Fracture mechanics of PVDF polymeric material : specimen geometry effects

International audiencePolyvinylidene fluoride (PVDF) is a semi-crystalline polymer that has been widely studied for structural applications, because it exhibits good mechanical properties and chemical resistance. During viscoplastic deformation, the material whitens after the onset of necking due to nucleation and growth of voids. Mechanical tests on cracked specimens show the crack instability on DENT specimens whereas stable crack growth on SENB specimens is also encountered. In the global approach of non linear fracture mechanics framework, the two-parameter approach indicates that according to the opening stress state in the remaining ligament, the crack growth can be more or less stable. Namely, tensile crack specimens such as DENT favour instabilities whereas bending specimens (like SENB) are proved to present stable cracking. This work deals with computing Q-stresses on DENT and SENB specimens with the help of FE modelling. The material toughness JIC is then determined by means of experimental data gathered with both specimens. JIC depends on the specimen geometry. A JIC-Q curve is then plotted for the PVDF material at 20°C

Toughness improvement of polyamide 11 assessed via quasistatic tensile tests on notched round bars

International audienceToughening a polymeric material by adding rubber particles is a common technique used forglassy polymers and measured by means of Charpy or Izod impact tests. These latter aredetermined under dynamic conditions, thus at high strain rate or equivalently at very lowtemperature for polymeric materials.This work is devoted to three grades of semi-crystalline PolyAmide 11 (PA11), used in alarge number of engineering components. Service conditions are close to the glass transitiontemperature. Therefore, tests with quasi-static loading are expected to be more appropriate.The aim of the study is to find a methodology allowing the quantification of toughnessimprovement. To this end, an experimental database was constituted. Three grades of PA11were involved, that enable to analyze effects of: i) aging, ii) addition of a plasticizer iii)addition of rubber particles. Tensile tests were then carried out on notched round bars.Trends of load versus notch opening displacement curves are investigated. Longitudinalcross-sections on specimens issued from interrupted tests and fracture surfaces wereexamined in order to study deformation and damage mechanisms. An attempt is made to linktoughness improvement to the increase in global fracture energy

Damage and fracture of PVDF at 20°C

International audienceFlexible oil pipelines are multilayered structures used for the transport of crude oil or natural gas from the seabed to the surface in offshore oil fields. Because of severe service conditions, composite structure made of metallic and polymeric layers must be used. PVDF is a good candidate as it accommodates tensile and flexural deformations and guarantees watertightness. The present paper focuses on the mechanical properties of PVDF at 20°C. Several specimens with different geometries were tested: notched specimen and cracked specimen under tension. Fracture surfaces were examined to determine fracture mechanisms. Based on mechanical testing and microscopic observations, a modified Gurson-Tvergaard-Needleman model for semi-crystalline polymers is proposed. The model allows to represent the non-linear behavior and the cracking of polymer structures

Continuous Bamboo Fibers/Fire-Retardant Polyamide 11: Dynamic Mechanical Behavior of the Biobased Composite

A biobased composite was generated from bamboo fibers (BF) and a polyamide 11 (PA11) matrix. In order to fulfill security requirements, a PA11 already containing a flame retardant (FR) was chosen: This matrix is referred as PA11-FR. In this work, the effects of flame retardant (melamine cyanurate) on the composite properties were considered. In the calorimetric study, the glass transition and melting temperatures of PA11-FR were the same as those of PA11. The melamine cyanurate (MC) had no influence on these parameters. Thermogravimetric analysis revealed that PA11-FR was less stable than PA11. The presence of MC facilitated thermal decomposition regardless of the analysis atmosphere used. It is important to note that the presence of FR did not influence processing conditions (especially the viscosity parameter) for the biosourced composite. Continuous BF-reinforced PA 11-FR composites, single ply, with 60% of fibers were processed and analyzed using dynamic mechanical analysis. In shear mode, comparative data recorded for BF/PA11-FR composite and the PA11-FR matrix demonstrated that the shear glassy modulus was significantly improved: multiplied by a factor of 1.6 due to the presence of fibers. This result reflected hydrogen bonding between reinforcing fibers and the matrix, resulting in a significant transfer of stress. In tensile mode, the conservative modulus of BF/PA11-FR reached E’ = 8.91 GPa. Upon BF introduction, the matrix tensile modulus was multiplied by 5.7. It can be compared with values of a single bamboo fiber recorded under the same experimental conditions: 31.58 GPa. The difference is partly explained by the elementary fibers’ lack of alignment in the composit

Accounting for porosity, time and temperature dependency in fracture mechanics concepts on polyvinylidene fluoride material

International audienceThe polyvinylidene fluoride (PVDF) under study is a semi-crystalline polymer that exhibits sensitivity of mechanical properties to both strain rate and temperature. Furthermore, this material is subjected to a significant cavitation during deformation. A comprehensive experimental database was built in order to analyze the fracture behaviour in the ductile to brittle transition domain. Tensile tests were carried out on smooth and notched specimens at temperatures ranging from −50 °C to 20 °C. The results were used to determine temperature-dependent material parameters by using the mechanics of porous media. The obtained set of parameters was validated on two kinds of pre-cracked specimens, by using the local approach of fracture mechanics. With the help of a finite element code, both global and local approaches of fracture mechanics were shown to complement one another: whereas classical formulae of J-integral fail to characterize crack initiation for this PVDF, the present methodology allowed the plot of J1C values with respect to temperature

Fracture mechanics concepts applied to PVDF polymeric material exhibiting porosity, time and temperature dependency

International audienceThe polyvinylidene fluoride ....

Experimental and three-dimensional finite element study of scratch test of polymers at large deformations

International audienceAn experimental and numerical study of the scratch test on polymers near their surface is presented. The elastoplastic response of three polymers is compared during scratch tests at large deformations: polycarbonate, a thermosetting polymer and a sol-gel hard coating composed of a hybrid matrix (thermosetting polymer-mineral) reinforced with oxide nano-particles. The experiments were performed using a nanoindenter with a conical diamond tip having an included angle of 30 deg and a spherical radius of 600 nm. The observations obtained revealed that thermosetting polymers have a larger elastic recovery and a higher hardness than polycarbonate. The origin of this difference in scratch resistance was investigated with numerical modelling of the scratch test in three dimensions. Starting from results obtained by Bucaille (J. Mat. Sci., 37, pp. 3999-4011, 2002) using an inverse analysis of the indentation test, the mechanical behavior of polymers is modeled with Young's modulus for the elastic part and with the G'sell-Jonas' law with an exponential strain hardening for the viscoplastic part. The strain hardening coefficient is the main characteristic parameter differentiating the three studied polymers. Its value is equal to 0.5, 4.5, and 35, for polycarbonate, the thermosetting polymer and the reinforced thermosetting polymer, respectively. Firstly, simulations reveals that plastic strains are higher in scratch tests than in indentation tests, and that the magnitude of the plastic strains decreases as the strain hardening increases. For scratching on polycarbonate and for a penetration depth of 0.5 μm of the indenter mentioned above, the representative strain is equal to 124%. Secondly, in agreement with experimental results, numerical modeling shows that an increase in the strain hardening coefficient reduces the penetration depth of the indenter into the material and decreases the depth of the residual groove, which means an improvement in the scratch resistanc

Influence of the rheology of polymers on their scratch resistance: Experimental and numerical simulation studies = Influence de la rheologie des polymeres sur leur resistance a la rayure: Etude experimentale et par simulation numerique

International audienc

Rapid crack propagation in PA11: An application to pipe structure

Dynamic fracture mechanism in Polyamide 11 (PA11) material has been described at laboratory scale to access to an intrinsic material parameter. A liquid transportation application is considered with polymer pipes. A preliminary numerical analysis of the rapid crack propagation (RCP) in polymer pipe is firstly realised. Two boundary conditions, imposed displacement or pressure, are numerically investigated. The work of external forces is not negligible for pressurized polymer pipe. A reliable estimate of the dynamic energy release rate GId is in this last case not guaranteed. To limit unwanted structural effects a specific experimental device has been used to ensure a permanent regime of RCP in Pre-Stressed Pipe Specimen (PS2). Experimental dynamic fracture tests are realised with Polyamide 11 PS2. Dynamic instabilities inducing “ring-off” and “snake” mechanisms which could appear during full-scale test are not observed with this new test. A finite element procedure is used to estimate the material toughness GID of PA11. Knowing the crack tip location during RCP inertia effects (i.e. kinetic energy) are quantified. The mean crack tip velocity is observed not to change in PA11 whatever the crack configuration (branching or not). This velocity is known to be the crack branching velocity (≈0.6cR). The average dynamic energy release rate 〈GID〉 is equal to 1.5± 0.1 kJm−2 at the crack branching velocity. The nontrivial fracture surface roughness is observed with a scanning electron microscope

Temperature dependent mechanical behaviour of PVDF: Experiments and numerical modelling

International audienceThe mechanical behaviour of Polyvinylidene Fluoride (PVDF) is analysed. To this end, tensile tests are performed on both smooth and notched specimens, for several values of the notch radius in order to set specific values of the stress triaxiality ratio in the net section. Tests were performed at various temperatures and at various strain rates. Experimental data together with fracture surface examinations by SEM allow the dependence of deformation and void growth processes on strain rate and temperature to be investigated. This experimental work was carried out in order to test the mechanics of porous media model. For each investigated temperature, constitutive relations take both porosity and strain rate sensitivity into account. The model is proposed for deformation leading to crazing. The material coefficients are optimised by imposing a continuous dependence on temperature